Production of thermoset pressure gelation castings

ABSTRACT

A process for making a cured plastics moulding by introducing a curable resin composition to a heated mould and supplying further resin under pressure to compensate for shrinkage of the composition including heating the curable resin composition either before entry or as quickly as possible on entry into the mould, or both to a temperature which permits the satisfactory filling of the mould before gelation and subsequent progressive gel from the point furthest from entry, back to the point of the mould entry, and wherein the entry parts of the mould are heated to a temperature at least as high as the temperature of the final fill point of the mould.

BACKGROUND OF THE INVENTION

The automatic pressure gelation technique (APG) for moulding articleswith thermosetting resins is well known and consists essentially of thesupply of liquid curable resins under pressure into a mould and ensuringthat the pressurised liquid supplied from outside the mould remains incontact with the liquid resin in the mould whilst it gels or solidifiesfrom the furthest point or points from mould entry and or the last pointto fill back to the mould entry. By ensuring this applied pressure ismaintained on the liquid resin system and the gel progression followsthis pattern excellent void and shrinkage free mouldings are obtained bycompensating for polymerisation shrinkage with further liquid suppliedfrom the external pressurised source.

This process is well proved in practice for the manufacture of bothcomplex and simple shaped mouldings alike. The process can be used forlong production runs of mouldings of high quality with very low rates ofrejection. The production of such moulding has been described in PatentsEP 0 333 456B1 and U.S. 5,064,585 and in these consists essentially offilling moulds that have temperature within the mould set in the rangeof from around 120° C. to 160° C. at the point of entry to 130° C. to170° C. at the furthest point from entry where the furthest point isalways the higher and with the ideal temperature at points furthest fromresin entry being approximately 10° C. higher than the temperatures atthe entry point. Such positive temperature gradients had been stated tobe important in order to obtain excellent mouldings from epoxy/anhydridecuring compositions which are particularly suited to this process. Thetemperatures had been chosen to achieve a linear gradient in themoulding operation to ensure the desired gel sequence as stated

However, there is a great need to speed this moulding process to achievethe maximum possible use of capital equipment such as moulds and pressmachines, to reduce labour cost per unit and to approach or to exceedthe speed of producing the equivalent item made by thermoplasticinjection or dough or sheet moulding compound matched tool moulding. Theconditions taught under the patents referenced above do not permit thisspeed of production with the desired type of resin system.

In order to increase the speed of the process, higher temperatures canbe used. However using the known process where a temperature gradient ismaintained the temperature gradient at the hotter end of the mouldcannot be increased too much as this will result in decomposition of theresin hardener system.

SUMMARY OF THE INVENTION

We have now found that by eliminating the temperature gradient, or evenreversing it so that the entry point is at a higher temperature than thefurthest point from entry, a decrease in the moulding cycle time can beachieved without decomposition of the resin and without changing thedesired gel sequence. The net result is that for any given temperatureat the furthest point from entry and for any given resin system, thespeed of the process is increased.

Accordingly, the present invention provides a process for making a curedplastics moulding by introducing a curable resin composition to a heatedmould and supplying further resin under pressure to compensate forshrinkage of the composition which comprises heating the curable resincomposition either before entry or as quickly as possible on entry intothe mould, or both, to a temperature which permits the satisfactoryfilling of the mould before gelation and subsequent progressive gel fromthe point furthest from entry back to the point of mould entry, andwherein the entry parts of the mould are heated to a temperature atleast as high as the temperature of the final fill point of the mould.

The temperatures actually employed may be any which allow the desiredsequence of gelation to take place provided degradation of the resinsystem itself does not occur. Thus for very reactive systems of whatevercomposition e.g. aliphatic amine cured epoxy resins, acrylic resins,vinyl esters polyurethanes or unsaturated polyesters it could be 100°C.-150° C. or even less, whereas for anhydride cured epoxy resins itmight be as high as 200° C. or even higher.

In carrying out the process of the invention the resin system should beheated as quickly as possible to the desired temperatures either beforeentering the mould or once it has entered a mould tool and left theexternal feed supply. In a preferred embodiment, the resin compositionis supplied to the mould pre-heated to or above the mould temperature atthe entry to the mould. To do this the mould temperature nearer to thecooler resin external inlet should be as high as possible, at least ashigh as, and may be even higher than that furthest away from the inlet.By adopting this approach the resin system is heated on entry as quicklyas possible to the desired gelling temperature and maintained close tothat temperature as the resin system progresses to the furthest point ofthe mould or the last to fill which it must reach before gelationcommences, whence the gel front moves progressively back to the entrypoint.

This approach is opposite to all past teaching for such a process as itrequires zero or even negative temperature gradients in the mould fromthe point of entry to the furthest point of the mould or the last tofill.

DETAILED DESCRIPTION OF THE INVENTION

It has also been well documented and long believed that in order toproduce mouldings with a cosmetically high definition face it isnecessary to heat the section of the mould providing this face to ahigher temperature than the section of the mould producing the reverseface. This moulding technique is of general use but particularly sowhere large or complex mouldings are produced and where cosmeticappearance is of great importance.

We have now found that excellent mouldings can be produced employing therapid in-mould heating principles of this invention where there is notemperature differential from mould half to corresponding mould halfacross the moulding regardless of temperature variations within theindividual mould halves. In addition, it has been found that byoperating with zero or very low temperature differentials across amoulding, particularly those with a relatively low through mouldingthickness, very low stress levels are achieved. This is especiallyevidenced by the production of pressure gelled sheets where zero or lowacross mould temperature differentials result in essentially unbowed andhence unstressed flat sheets.

Thus the present invention whilst not precluding across mouldingtemperature differentials does offer the advantages of the rapidproduction of high quality essentially stress free mouldings when zeroor near zero corresponding mould half temperatures differentials areemployed. This can be particularly so with both mould halves at aconstant temperature but also with a higher temperature near the inletof the mould tool and a lower temperature at parts furthest from theinlet or the last to fill.

The process of the invention may be used for the production of mouldingshaving thin or thick walls (cross sections). They are also particularlysuitable for the production of mouldings having a large surface area, atleast one large linear dimension or a complex shape. The compositionsmay be used, for instance, in the moulding of domestic sanitary waresuch as sinks, baths, shower trays and basins, sheet slabstock for usein the production of articles such as domestic workshops, chemicallyresistant containers such as tanks and parts such as pumps, valves andpipes for handling corrosive fluids and impact-resistant mouldings foruse in cars and other vehicles, and electrical applications.

Anhydride cured epoxy resins have typically been used in the moulding ofsuch diverse items as electrical insulators and domestic hardware,yielding excellent products but with cycle times varying from around 10minutes to 60 minutes or even longer.

Typically, conventional moulding conditions for castings of section of1-2 cms thickness or less would comprise a temperature gradient set inthe mould of from 140° C.-160° C. at the points of entry up to 150°C.-170° C. at the point furthest from entry. Such conditions would givean in mould cycle of around 9-15 minutes with an epoxy anhydridecomposition. This is not as fast as thermoplastic injection moulding orsome matched tool dough moulded compound (DMC) or sheet moulded compound(SMC) moulding processes.

We have now found surprisingly that by using the process of theinvention for fast cycle pressure gelation moulding and reversing thepreviously held conventions and practices excellent mouldings can beproduced in little more than the mould fill time using constant or evennegative temperature gradients in the mould from entry to last fill orfurthest parts of the mould from entry.

For instance, in EP 0333456B1, the cycle time for making domestic sinksis at least 15 minutes. The actual time depends on the size of theproduct made. However using the process of the invention and the sameepoxy formulations as in the EP specification we have produced kitchensinks with a total fill, gel and demould cycle of less than 12 minutes,often less than 10 minutes.

The mould gel time may be less than 6 minutes, preferably less than 4minutes.

Excellent mould cycle times can be achieved with an epoxy anhydridesystem designed for fast gel at high temperatures with no degradationwith mould settings at a constant temperature of from 175° C. to 200° C.or with entry temperatures of 170°-220° C. e.g. 190° C.-200° C. andfurthest point from entry temperatures of 170° C.-200° C., e.g. 175° C.

Suitable epoxy resin systems for use in the process of the invention aredescribed in EP 0533465, EP 0599600 and EP 0604089.

Thus, by adopting these techniques which previously would have beenthought of as quite unsuitable we have achieved a major advantage in thePressure Gelation moulding process and can reduce mould cycles to becompetitive with every other process for the manufacture of practicallysimilar size and shaped articles where exothermic effects due toexcessive wall or bulk thickness do not cause gelation sequenceirregularities.

It will be clear to those familiar with the technology that the samenon-classical mould temperature settings could be used to achieve thesame result for other and more reactive thermosetting systems wherelower temperature ranges will be more appropriate.

The invention is illustrated by the following Examples.

EXAMPLE 1

Kitchen sinks are made using an epoxy resin composition containing 100parts liquid modified bisphenol A epoxy resin with an epoxy content of5.2 mol/kg, 250 parts finely divided silica, 84 parts of a liquidanhydride hardener and latent accelerators.

The mixture at 50° C. is stirred under vacuum and then injected at100-250 kPa pressure into the bottom edge of a two-part steel mouldproviding a shaped cavity of overall height 900 mm, width 600 mm anduniform thickness 0.6 mm having a capacity of about 5.6 litres.

The temperature of the mould is at substantially 175° C. over each halfof the mould. The sinks are made with a cycle time of about 6 mins. 20secs.

EXAMPLE 2

Flat sheets are made using an epoxy resin composition as in Example 1.The mould size is 700×500×6 mm and the injection temperature is 55° C.Both halves of the mould are heated to an entry temperature of 148° C.and the temperature at the point furthest from entry is 146° C.

Flat sheets are obtained with a mould cycle of 5-6 mins and which showedno distortions after a post cure cycle of 3 hours at 135° C.

We claim:
 1. A process for making a cured plastics moulding by introducing a curable resin composition into a heated mould and supplying additional curable resin composition into the heated mould under pressure to compensate for shrinkage of the composition, which process comprises heating the curable resin composition before entry into the mould, as quickly as possible upon entry into the mould, or both, to a temperature which permits the satisfactory filling of the mould before gelation and subsequent progressive gelling from the point furthest from entry back to the point of mould entry, and wherein the entry parts of the mould are heated to a temperature at least as high as the temperature of the point furthest from the entry of the mould and in which the resin composition is supplied to the mould preheated to or above the mould temperature at the entry to the mould.
 2. A process as claimed in claim 1 in which the curable resin composition is polyester, vinyl ester, acrylic, urethane or epoxy resin.
 3. A process as claimed in claim 1 in which the curable resin compositions is an epoxy/anhydride system, the mould temperature at the entry point is from 170° C.-220° C. and the temperature at the final fill point is from 170° C.-200° C.
 4. A process as claimed in claim 3, in which the temperature at the entry point is from 190° C.-200° C. and the temperature at the final fill point is 175° C.
 5. A process as claimed in claim 3, in which the mould temperature is at a constant temperature of from 175° C.-200° C. 